An uninterruptible power supply (UPS) system can be implemented to ensure constant power supply to a load even when an input power source fails. To accommodate the power requirement of certain loads, such as loads with high power requirements, a UPS system with energy storage, such as a battery or flywheel (shown in FIG. 1), can be employed to bridge the time between grid power loss and generator start-up. A battery system offers lower cost and higher energy density, but is unregulated and limited in life, in addition to having reliability issues. A flywheel system offers adequate energy to bridge the power for a generator start, while offering long operating life and high reliability. The flywheel system interfaces with the input power source, or power grid (e.g., a plant wide, citywide, statewide and/or nationwide utilities power transmission grid), and the load. The electrical power supplied by the flywheel system may need to be converted to meet the power quality requirements of the load. An interface that provides the required power conversion components may be costly.
For example, FIG. 2 depicts an example configuration 200 of components used in an interface between a flywheel motor/generator system 210 and a power grid 230 and load 235. In the illustrated example, the flywheel system 210 interfaces on the direct current (DC) bus 240 of a dual conversion UPS system. A passive rectifier 220 of the UPS converts incoming alternating current (AC) from a power source, such as power grid 230, into direct current. An active inverter 225 is used to convert the direct current from the passive rectifier 220 back into alternating current to power the load 235. For the flywheel system, an active bidirectional converter 215 is used to both (1) convert DC from the passive rectifier 220 into alternating current to power the motor/generator of the flywheel system 210, and (2) convert AC from the flywheel system 210 to DC to power the load when the power source 230 is not available. The power conversion components 215 and 225 illustrated in FIG. 2 all carry full rated power of the input rectifier 220 and must have a full load power rating. Such full power active components are typically expensive. In addition, the bidirectional converter must have very high switching frequency to minimize flywheel losses, thus making it even more costly.
FIG. 3 depicts another example configuration 300 of components used in an interface between a flywheel motor/generator system 310 and a power grid 330 and load 335. An active inverter 320 coupled to an active bi-directional converter 315 provides the interface between the flywheel system 310 and the power grid 330 and load 335. Here, the active inverter 320 and active converter 315 carry full power from the power grid 330 and must have a full load power rating. Again, such full power components are typically expensive.